Some therapeutic molecules can be transported through the skin. The principal barrier to the transport of molecules through the skin is the stratum corneum (the outermost layer of the skin). Devices or “microarrays” equipped with one or more microneedles have been disclosed for use in the delivery of active agents and other substances through the skin and other surfaces. In use, the microarray is pressed against the skin with sufficient force to cause the microneedles to puncture the stratum corneum and thereby create a plurality of microscopic slits through which the transdermal delivery of one or more active agents may be accomplished. Similarly, fluid sampling may be facilitated by the creation of such microscopic slits.
In delivering a therapeutic molecule or active agent through the stratum corneum, a microarray can be associated with a fluid reservoir that can temporarily retain a liquid formulation of active agent prior to its delivery through mammalian skin. In some constructions, the microneedles are hollow structures that provide a path for liquid to flow directly from the fluid reservoir through the microneedles and through the skin. In other constructions, an active agent may be coated on the outer surfaces of a microarray so that the active agent on the surfaces of the microneedles is exposed to interstitial fluids as soon as the microneedles have pierced through the stratum corneum. Once the microneedles are in contact with interstitial fluid, the coating can dissolve. The goal of such a configuration is to dissolve the active agent into the interstitial fluid so that it can enter the body to perform its therapeutic function.
However, active agents can have a strong affinity to the microneedle materials, and such an affinity can interfere with the disassociation of active agent from a microneedle surface. Although the concentration of active agent(s) in a coating can be increased to compensate for the inability to disassociate all of the active agent from the microneedles, the amount of active agent left on the microneedles of a used microarray is wasteful and can present disposal issues.